K:JNWTS 28/3 (December 2013): 17
There, however, still remains one question. If God rendered to David a just recompense, it may be said, does it not seem, when he shows himself liberal towards his people, that he is so in proportion as each of them has deserved? I answer, When the Scripture uses the word reward or recompense, it is not to show that God owes us any thing, and it is therefore a groundless and false conclusion to infer from this that there is any merit of worth in works. But God, as a just judge, rewards every man according to his works, but he does it in such a manner, as to show that all men are indebted to him, while he himself is under obligation to no one. The reason is not only that which St. Augustine has assigned, namely, that God finds no righteousness in us to recompense, except what he himself has freely given us, but also because, forgiving the blemishes and imperfections which cleave to our works, he imputes to us for righteousness that which he might justly reject. If, therefore, none of our works please God, unless the sin which mingles with them is pardoned, it follows, that the recompense which he bestows on account of them proceeds not from our merit, but from his free and undeserved grace (Commentary on the Book of Psalms , 1:280 on Ps. 18:20).
 We owe this passage to Robert Van Kooten, who spotted it during research for a series of sermons on the Psalms.